In recent years, a high-voltage electrical storage device having high energy density has been desired as a power supply for driving an electronic device. A lithium-ion battery, a lithium-ion capacitor, and the like have been expected to be such an electrical storage device.
An electrode used for such an electrical storage device is normally produced by applying a composition (electrode slurry) that includes an active material and a polymer that functions as a binder to the surface of a collector, and drying the applied composition. The polymer used as a binder is required to exhibit a capability to bind the active material, a capability to bind (bond) the active material and the collector, scratch resistance when winding the electrode, fall-off resistance (i.e., a fine powder of the active material or the like does not fall off from the layer (film) formed of the composition (hereinafter may be referred to as “active material layer”) due to cutting or the like), and the like. When the polymer meets these requirements, it is possible to produce an electrical storage device that has a high degree of freedom with respect to the structural design (e.g., electrode folding method or winding radius), and can be reduced in size.
It was experimentally found that a capability to bind the active material, a capability to bind the active material and the collector, and the fall-off resistance have an almost proportional relationship. Note that these properties may be comprehensively referred to herein as “adhesion”.
In recent years, use of a material having a high lithium occlusion capacity has been studied in order to implement an increase in output and energy density of the electrical storage device. For example, a technique has been studied that improves the lithium occlusion capacity by utilizing highly crystalline graphite as the active material to implement a capacity close to the theoretical lithium occlusion capacity (about 370 mAh/g) of a carbon material. JP-A-2004-185810 discloses a technique that utilizes a silicon material having a maximum theoretical lithium occlusion capacity of about 4200 mAh/g as the active material. It is considered that the capacity of the electrical storage device is significantly improved by utilizing an active material having a high lithium occlusion capacity.
However, an active material that utilizes a material having a high lithium occlusion capacity significantly changes in volume along with occlusion and release of lithium. Therefore, when a known electrode binder is applied to a material having a high lithium occlusion capacity, the active material may be removed due to a decrease in adhesion, and a significant decrease in capacity occurs along with charge and discharge, for example.
In order to improve the adhesion of the electrode binder, JP-A-2000-299109 and WO2011/096463 disclose a technique that controls the composition of the binder, and JP-A-2010-205722 and JP-A-2010-3703 disclose a technique that improve the above characteristics by utilizing a binder that includes an epoxy group or a hydroxyl group, for example. JP-A-2011-192563 and JP-A-2011-204592 disclose a technique that restrains the active material using the rigid molecular structure of polyimide to suppress a change in volume of the active material.
In order to use such an electrode binder in the actual production line, the electrode binder is required to exhibit excellent charge-discharge characteristics, excellent adhesion, and excellent storage stability (i.e., the electrode binder does not change in quality (e.g., due to precipitation of the polymer) even when the electrode binder is stored for a long time in a warehouse or the like). JP-A-2012-9775 discloses a technique that adds a preservative in order to improve the storage stability.